1. Field of the Invention
This invention relates to a control device for a hybrid electric vehicle, and particularly a control device for a hybrid electric vehicle arranged such that a driving force of an engine and a driving force of an electric motor can be transmitted to the driving wheels of the vehicle.
2. Description of the Related Art
A so-called parallel hybrid electric vehicle capable of transmitting a driving force of an engine and a driving force of an electric motor to the driving wheels of the vehicle has been developed and already come into practical use.
Such a parallel hybrid electric vehicle is proposed in Japanese Unexamined Patent Publication No. 5-176405 (hereinafter referred to as Patent Document 1), for example. The parallel hybrid electric vehicle proposed comprises a clutch for mechanically connecting/disconnecting the engine and an automatic transmission, and the rotary shaft of the electric motor is connected between the output shaft of the clutch and the input shaft of the automatic transmission.
In the hybrid electric vehicle as shown in Patent Document 1, when the vehicle is to start traveling, the clutch is disengaged and the electric motor is operated as a motor with power supply from a battery, so that the vehicle starts traveling only on the driving force of the electric motor. In the traveling of the vehicle after the start, the clutch is engaged, so that the driving force of the engine can be transmitted to the driving wheels through the transmission.
Meanwhile, in deceleration of the vehicle, the electric motor is operated as an electric generator to produce a regenerative braking force, and regenerative braking energy is converted into electric power to charge the battery.
When the hybrid electric vehicle is decelerating with the accelerator pedal released and with the brake of the vehicle not operating, it is desirable to control the electric motor and the engine to produce a deceleration almost equal to the deceleration which a vehicle of a similar level but provided with only an engine as a power source produces in similar deceleration. By controlling the electric motor and the engine like this, giving a uncomfortable feeling to the driver can be avoided, and lowering of a driving feeling can be prevented.
A control device for a hybrid electric vehicle designed to control the engine and the electric motor to obtain a desired vehicle deceleration is proposed in Japanese Unexamined Patent Publication No. 2000-224713 (hereinafter referred to as Patent Document 2), for example.
The hybrid electric vehicle disclosed in Patent Document 2 has an arrangement such that an electric motor is disposed between a torque converter and an engine, and the output shaft of the engine is connected with the rotary shaft of the electric motor.
In such an arrangement as the hybrid electric vehicle of Patent Document 2, the driving shaft of the engine is always connected with the rotary shaft of the electric motor. Thus, part of rotational energy of the driving wheels in vehicle deceleration is always consumed by the engine, so that the energy recovery efficiency lowers by the amount consumed.
In order to prevent the engine from producing a decelerating torque to increase the energy recovery efficiency in vehicle deceleration, the engine needs to be operated by supplying fuel. In this case, however, fuel is consumed in order to increase the energy recovery efficiency, which leads rather to degradation in fuel economy.
In the hybrid electric vehicle in which a clutch for mechanically connecting/disconnecting the engine and the automatic transmission is provided and the rotary shaft of the electric motor is connected between the output shaft of the clutch and the input shaft of the automatic transmission, as disclosed in Patent Document 1, it is possible to decelerate the vehicle only by the regenerative braking torque of the electric motor, and accordingly, it is possible to increase the energy recovery efficiency. The electric motor has, however, a characteristic that the decelerating torque produced by the electric motor tends to decrease as the revolution speed increases. Thus, an attempt to obtain a sufficient decelerating torque from the electric motor even at high revolving speeds leads to problems such as an increase in size of the electric motor, an increase in weight of the vehicle, and an increase in the space occupied by the electric motor within the vehicle.